gmessages: Flush output stream after logging messages

When debug output is enabled then certain messages will be logged to
stdout. stdout however is block buffered by default when it isn't going
to a TTY meaning that debug logging will not be flushed out properly
when it is being redirected. One example of this happening may be tests
that rely on parsing g_debug messages.

Adding an explicit fflush ensures all log messages will reach the output.

https://bugzilla.gnome.org/show_bug.cgi?id=792432